Step one, secure a piece of glass thick enough to withstand the following - and use caution, please.
Step two, secure a supply of DAP acrylic caulk in the available color of your choice (probably clear for water effects). Don't even think about using silicon caulk - you will regret it.
Step three, secure a roller for spreading out the caulk as thin as you like.
Step four, place tape margins on the underside of the glass to control width.
Step five, secure a good quality box knife with extra blades (always use sharp blades) - and, again, use caution, please.
Step six, roll out the caulk as thin as you dare (it can go pretty thin). Clean roller as you go.
Step seven, allow to dry for a bit.
Step eight, cut as needed to achieve the desired piece.
Step nine, carefully peel up the piece or pieces and set aside to allow to really get very dry.
Step ten, spray paint with a flat color, both sides (prevents stickiness in the future) and allow to dry.
Step eleven, paint the terrain pieces as desired, add flocking and other bits as desired, allow to dry.
Step twelve, play with your new terrain in a game.
There you have it, a twelve step program with not a drink in sight!
By the way, the reason for the glass is to provide a nice, smooth, even bottom to the pieces. It will take a bit of practice and you will probably "ruin" a few pieces before you get the hang of it.
Final step, pay the bill I'm about to send you - where is that I send it again?!